The present invention relates to a particle accelerating structure, and more particularly to a structure suitable for both increasing the energy of and raising a repetition frequency of a particle beam periodically emitted from the structure. The particle stream emitting from an RF accelerating structure may be used for multiple purposes, for example medical treatment, non-destructive investigation of solid objects, and the like.
An example of a particle accelerator suitable for application of the present invention is a radio-frequency (“RF”) accelerator using a photocathode which typically comprises a conductive housing defining a cavity, a photocathode for emitting photoelectron into the cavity, and a wave guide for generating an RF electric field in the cavity. As light is periodically applied to the photocathode, photoelectrons are emitted into the cavity intermittently. These photoelectrons are converged and accelerated by an RF electric field generated in the cavity. The RF electric field is applied synchronously with application of light to the photocathode. A typical RF accelerator is described in U.S. Pat. No. 6,094,010 to Washio, which is incorporated herein by reference.
Such accelerating structures generally include a housing made from a conducting material such as copper. The housing defines a cavity. A photocathode is mounted on an inner surface of the housing. Into the cavity is fed light (laser) via a window, and illuminates the surface of the photocathode. Photoelectrons are emitted from the photocathode into the cavity. Such a housing may include one or more cells, dividing the cavity into a plurality of sub-cavities which are separated from each other by toroidal shaped discs (known in the art as “irises”). The sub-cavities are sized and configured to resonate in a particular harmonic mode which corresponds to the frequency of a particular electromagnetic field induced in the irises, with the result that a strong longitudinal electric field is generated along a longitudinal central axis of the housing.
Once a longitudinal electric field has been established in this way, the photoelectrons are accelerated along the longitudinal axis to emerge from an exit port. The resulting stream of photoelectrons may used for any of the multiple purposes known in the art.
In general, it is desirable to operate an RF accelerating structure at the highest power possible. Very high duty factor, high gradient photo-injectors and RF cavities in general are a critical component of the next generation of applications in high energy electron beam-based physics. Today, there is a compelling need for these applications, which include linear colliders, x-ray free-electron lasers, inverse Compton scattering sources, as well as associated imaging or analysis applications of interest to homeland security. The key issue for high average power, normal conducting, photo-injectors and RF accelerating structures is to effectively cool the housing structure.
Thermal management of very high duty factor, high gradient RF structures is crucial to their performance. A significant percentage of the applied RF power is deposited on the walls of the housing in specific locations depending on the magnitude and the direction of the electromagnetic fields in that location. This heating presents significant thermal engineering problems. The large amount of power dissipated in the structure can cause “hot spots” and local thermo-mechanical distortions which may lead to detrimental changes in RF properties and beam quality. One of the most challenging parts of an accelerator housing to be cooled are the “irises” which protrude into the cavity of the housing. Another challenging aspect to be cooled is the so-called RF coupler, which is a thin walled interface between the waveguide and the cavity.
Accordingly, the problem of thermal gain has been approached by providing channels within the housing structure, and forcing water to flow through the channels in combination with cooling the water on the outside of the housing by conventional heat dissipation means such as by radiator. However, the prior art is limited in the method for creating, and configuring the channels used for cooling the housing structure.
One method currently used to incorporate cooling channels into RF structures is achieved by drilling elongate cylindrical holes into the structure for example, as described in U.S. Pat. No. 6,094,010 to Washio, which is incorporated herein by reference, and where it is specifically described how cylindrical holes are provided to cool the irises of an RF accelerator. It will be readily understood that because these channels are drilled, they are limited to linear configurations, and are connected to each other at sharp angles. It will be understood that this kind of configuration greatly limits the cooling uniformity and rate of cooling in that fluid flow is dramatically slowed by the sharp changes in direction (discontinuities in flow gradient), thereby reducing the rate at which heat can be extracted.
Another method that has been used to introduce cooling channels into RF structure is to braze sections of the structure together with pre-machined, curve shaped channels cut out in each section. However, brazing multiple components to form high gradient RF structures is a delicate and expensive step, and many braze cycles are needed to build an effective cooling structure. Moreover, the resulting structure is not uniform or homogeneous, which adversely affects the efficiency at which heat can be extracted from the housing by water in the channels.
Thus, there is a need for a method and structure for fabricating an RF housing having a channel system with gentle changes in direction, suitable for cooling the RF housing structure. There is a further need for an RF housing structure having such channel system, that has a uniform and homogeneous configuration, that is not a collection of components, with sections cut out, brazen together. The present invention addresses these and other needs.